captainsolo, it depends on which 5.1 track you listened to, hairy_hen describes the specifics of every mix on the first page, he did two of them for Empire and Jedi and also fixed a few glitches in the '93 PCM of Empire IIRC.
The echo on Luke's voice at the end of the duel is just one of the many things they screwed up in the '97 remix of Empire. It's heard in the original theatrical mixes as well as in the '93 remastering.